Micro-electro-mechanical systems (MEMS) gyroscopes and accelerometers, such as disclosed in U.S. Pat. Nos. 6,725,719, 6,859,751 and 7,406,867 (all incorporated herein by reference), are generally planar instruments. Due to their very small size, and materials and methods of construction, such MEMS inertial instruments are relatively sensitive to ambient temperature and other environmental factors, such as shock, vibration and acoustics. These sensitivities make them relatively unstable. Accordingly, it has been necessary to extensively test such instruments under a variety of conditions in order to develop a priori compensation schemes that can then be built into systems employing such instruments. This testing is time consuming and expensive, and also may not anticipate every possible condition that the instrument may be exposed to, which can lead to errors. The accuracy of MEMS gyroscopes and accelerometers depends on the stability of their bias. Bias is the non-zero instrument output in the absence of input. The bias has deterministic and random components. Each deterministic component can be related to a cause, which potentially can be controlled. Random components can be traced to a cause but cannot be controlled. The pertinent random component for this invention is the 1/f noise, one source of which is the intrinsic shot noise in the electronics. The deterministic components include turn-on to turn-on variations, offset and drift.
The problem is that under motion, the instrument bias is undistinguishable from the signal generated by the motion, hence the output is incorrect and the MEMS instrument is impractical and requires continuous correction to be useful.
A method for separating the bias from the signal is to spin the instrument so that its input axis is rotated relative to the body axis of measurement. The effect of the spin is to modulate the signal, with the signal maximum occurring when the two axes are aligned; this does not to modulate the bias, because it is not sensitive to the rotation. The bias can then be filtered and the modulated signal demodulated back to DC where the peak-to-peak of the modulated signal is proportional to the stabilized instrument signal. The modulation method is referred to as Phase Sensitive Demodulation (PSD) and is a standard practice applied for separating signals from noise especially if the signal is weak and imbedded in the noise. The spin is generally applied with a carousel having a built-in angle resolver. From the resolver a reference waveform is developed which is necessary for the demodulation step. The resolver is aligned with the instrument Input Axis, which is aligned with a body axis of the vehicle.
The gyro instrument measures rotation rate about the body axis. The accelerometer instrument measures acceleration along the body axis. The gyros and accelerometers make up an IMU. At least one gyro and one accelerometer is necessary for each body axis for the IMU to measure all six degrees-of-freedom of the vehicle motion. The Input Axes of the gyros and accelerometers are aligned with the three orthogonal axes of the IMU and the IMU axes are aligned with the three body axes of the vehicle. The body axes of the vehicle are the Pitch, Yaw and Roll Axes. One IMU alignment is for its Z-axis to align with the carousel spin axis and with the Roll Axis. For this case the Input Axes of the X, Y gyros and accelerometers are rotated about the spin axis and are therefore stabilized by the PSD method. The Z gyro and Z accelerometer are not.
Each of the X, Y instruments senses inputs of the Pitch and Yaw vehicle axes as they are rotated. Therefore the signals of the X, Y instruments are the sums of the separate Pitch and Yaw rates components. Two reference waveforms phased with the Pitch and Yaw axes are needed to separate the two components for each instrument.
The spin rate of the carousel is a determinant of the effectiveness of the method. A smaller instrument with greater bias instability will require a higher spin rate. The capability for the instrument to be unaffected during spin is an important requirement. In particular for MEMS instruments, which have low signals, the 1/f instability is relatively high. Therefore in addition to the reduction of the long-term bias drift, the reduction of the 1/f instability will improve the bias instability and best achievable resolution; two important performance parameters.